Selective amplifier



Jan. 6, 1942. J. PLEBANSKI SELECTIVE AMPLIFIER Filed Jan; 19, 1939 3 Sheets-Sheet 1 IN VENTOR.

ATTORNEY.

Jan. 6, 942. v J. PLEBANSKI 2,268,572

SELECTIVE AMPLIFIER Filed Jan. 19, 1939 3 Sheets-Sheet 2 INVENTOR. o f' -lebansfi ATTORNEY.

Jan. 6, 1942. PL AN 2,268,672

SELECTIVE AMPLIFIER Filed Jan. 19, 1939 3 Sheets-Sheet 3 Fig 16 INVENTOR.

gag 4P! 6 ba T's/f1: BY

Zy/ fl x ATTORNEY.

Patented Jan. 6, 1942 UNITED STATES SELECTIVE AMPLIFIER Jozef Plebanski, Warsaw, Poland, assignor to Radio Patents Corporation, a corporation New York Application January 19, 1939, Serial No. 251,682 In Poland May 24, 1938 12 Claims.

The present invention relates to amplifiers for alternating currents such as high frequency or medium frequency signal currents and more particularly to amplifiers of the retro-active type employing feedback of energy from a point at a higher oscillation level to a point at lower oscillation level of the amplifier.

An object of the invention is to obtain a desired selectivity with a limited number of circuit elements or amplifier stages in a receiver for high frequency or medium frequency signals compared with circuit arrangements at present known and used in the art.

Another object is to obtain an improved bandpass filter effect in a radio or other high frequency or medium frequency system serving to translate or amplify modulated signal energy.

A further object is the provision of a tuning system comprising multiple amplifying stages for receiving or translating modulated signal energy,

whereby all amplifying stages may be simultaneously tuned to a desired transmitting station by a limited number including a single tunable circuit.

Still a further object is the provision of a resonant multistage amplifier capable of being tuned to a desired transmitting station by adjusting a single selective circuit or network.

A further object is to increase both the selectivity of a tuned high frequency amplifier and to improve its performance by increased stability as well as to reduce distortion, background noise and other undesirable interference.

Another object is to provide a band-pass filter for use in a radio frequency amplifier or the like characterized by an improved frequency cut-off or increased selectivity compared with filters comprising an equal number of parts or circuits known in the art.

Another object is to provide a single-tunable circuit having a selectivity heretofore obtainable only with multiple circuits or sections to eliminate the necessity of ganging and aligning the circuits.

A further object of the invention is the employment of selective inverse feedback in a tuned radio frequency or medium frequency amplifier to improve both the operational stability and performance as well as the selectivity of the amplifier.

The above and further objects and advantages of the invention will become more apparent from the following detailed description taken with ref erence to the accompanying drawings forming part of this specification and wherein:

Figure 1 illustrates a simple selective inverse feedback amplifier constructed according to the invention,

Figure 2 shows resonance or selectivity curves illustrative of the function and operation of the circuit according to Figure 1,

Figures 3 and 4 represent, respectively, resonance curves and an improved selective feedback circuit of the type'according to Figure 1,

Figure 5 shows a further modification of an inverse feedback amplifier for obtaining a bandpass characteristic with sharply defined'cut-ofi frequencies,

Figures 6 to 9 are diagrams illustrating the function and advantages of Figure 5,

Figures 10 to 13 represent further "improved modifications of a selective inverse feedback receiver of the type according to Figures 1 and 4,

Figures 14 to 16 illustrate embodiments of the invention for tuning a plurality of amplifying stages by a single resonant circuit or tuning element.

Similar reference characters identify similar parts throughout the different views of the drawings.

Referring to Figure .1, there is shown an antenna input circuit for receiving high frequency signals from a transmitting station, said circuit comprising an antenna H! in series with a coupling or shorteningcondenser II and an input inductance [2, the latter being connected to ground M. In place of an antenna, input signal energy may be supplied by any other source such as a transmission line or the like as is understood. The inductance l2 which may be shunted by a fixed or variable condenser I3, is connected across the grid-cathode paths of a pair of triode valves I5 and I6 through grid coupling condensers l1 and I9 and grid leak resistances l8 and 20,- respectively. Valve 16 functioning in the example shown as a detector or demodulator has its anode connected to the positive terminal of ahigh tension source or B-supply indicated by the sign through a high frequency choke coil 28 and a coupling resistance 30. Items 3| and 32 are by-pass condensers for high frequency and 33 is a coupling condenser connected between the lower end of resistance and the detector output terminal I), The demodulated signals as obtained from terminals a, b may be fed to a further amplifier or a utilization circuit such as a reproducer or other translating device.

Valve l5 which is simultaneously excited from the input coupling coil l2 serves to supply inverse feedback energy and to this end has its anode connected to the pole of the high tension source through a high frequency coupling coil 29 on the one hand and to ground on the other hand through a feedback circuit, said feedback circuit including a blocking condenser 2| and a feedback coil 26 arranged in inductive coupling relation with the input induction coil l2 in the antenna circuit. Coils l2 and 26 are Wound in such a manner that the feedback currents induced in the coil l2 are in phase opposition to the signal current received therein from the antenna thereby reducing or cancelling the output at terminals a, b. In this manner even .very strong signals from a powerful local transmitter may be eliminated provided that the condenser l I has a sufficiently small capacity, in practice below about 50 mmf., and that sufficiently strong inverse feedback or a tight coupling between the coils l2 and 26 is employed. 7

In order to render the inverse feedback inefiective for the desired signals, there is further provided in series with the inverse feedback path a tunable rejector circuit comprising an inductance 23 shunted by a variable condenser 24. This rejector circuit is preferably magnetically screened or decoupled from the other circuit elements such as by enclosing it with a metallic screen indicated at 25. As is well understood, the circuit 23, 24 allows the passage of feedback currents of all frequencies except its resonant frequency, whereby the inverse feedback effect for this particular frequency is cancelled and signals of a desired frequency may be translated and impressed upon the grid of the amplifier l6. As pointed out, the condenser l3 may be omitted or a fixed condenser substituted therefor broadly tuning the input circuit in such a manner that the entire system may be tuned to a desired receiving signal by adjusting a single tuned circuit, i. e. the rejector circuit 23, 24 in the example illustrated.

In Figure 2 there are shown the resonance curves for a circuit according to Figure 1 in comparison, with the resonance curves for a customary positive feedback or regenerative receiver. In this figure which illustrates in a known manner the output response such as the current I plotted against frequency f, the solid curves A, B, C represent the characteristics of a customary positive feedback or regenerative circuit, curve A corresponding to the condition without regeneration and curves B and C corresponding, respectively, to lower and higher degrees of positive feedback or regeneration. As the degree of regeneration is increased, the signal response at the resonance frequency it increases, but the response to side frequencies remains still considerable. This characteristic constitutes a great disadvantage of regenerative circuits as is well understood by those skilled besides other well known drawbacks and defects of positive feedback or regeneration. The dotted curves illustrate the conditions for inverse or negative feedback proposed by the present invention. As'is seen, the response at resonance frequency remains approximately constant or is slightly decreased as the degree of negative feedback or degeneration increases, curve D showing the curve for a small degree and curve E for a high degree of inverse feedback. In the latter case, the cut-off at the ends of the frequency bands is considerably improved resulting in a more favorable, fiat top type bandpass characteristic as compared with the curves obtainable with positive feedback or regeneration. This characteristic of the invention enables the attainment of high selectivity with a single or a limited number of cascaded amplifying stages compared with the conventional selective amplifiers. It has been proved by experiments that at a frequency of about 200 kc. a distant station separated from a local transmitter by about 20 kc. could be received with full volume and substantially without interference by the local signal. The attenuation of the local station due to the inverse feedback was about 40 to 60 db.

In an arrangement according to Figure 1, a certain amount of positive feedback or regeneration usually occurs in addition to the inverse or negative feedback for the desired signal or receiving frequency to which the rejector circuit 23, 24 is tuned. According to a further feature of the invention, this positive feedback or regeneration is utilized and increased by providing additional positive reaction upon either the input or rejector circuits derived from a subsequent stage such as the main amplifying valve l6 as shown. In this manner the signal response may be increased while retaining the advantages of the inverse feedback; that is, improved cut-oif or selectivity in the manner described.

A circuit of this type is shown in Figure 4 which differs from Figure 1 by the provision of an additional feedback coil 36 in inductive coupling relation with the inductance 23 of the rejector circuit and inserted in the cathode lead of the valve l6 which latter in the example illustrated functions as a high frequency amplifier. For this latter purpose the anode of valve is is connected to cathode or ground through a blocking condenser 33 and a parallel tuned coupling circuit comprising an inductance 31 shunted by a condenser 38. The high potential side of the circuit 31, 38 is connected to the output terminal b through a coupling condenser 39 and its low potential side is connected to the cathode or output terminal a. From terminals 0, b the amplified high frequency energy may be fed to further amplifying stage or directly to a detector. According to a further modification, the positive feedback coil 36 may be coupled with the input inductance l2; 1. e. to feedback potentials applied to the grid input circuit as shown and described in connection with Figures 10, 11 and 16.

In Figure 3 there are shown the resonance curves for a receiver according to Figure 4. Curve A corresponds to the receiver without any feedback, curve D to a receiver with negative or inverse feedback alone and curve F to combined inverse and positive feedback such as shown in Figure 4. If the positive feedback currents are applied to the rejector circuit 23, 24 as shown, there will result an improved cut-off of the curve D in Figures 2 and 3, while applying the positive feedback currents to the input circuit or coil l2 will produce both improved cut-01f and signal strength increase as shown by curve F in Figure 3.

Experiments carried out by applicant have shown that by employing two tuned circuits 23, 24 and 31, 38 in a system according to Figure 4 followed by a detector and a two stage audio frequency amplifier, the sensitivity and selectivity obtained was comparable to a superheterodyne receiver of known construction comprising a substantially higher number of amplifying stages and circuit elements. If condenser I3 is'omitted, the inductance I2 is preferably chosen so as to be resonant, either due to its owndistributed capacity or by means of a fixed shunt condenser, to a wave length just below the shortest wave length to be received or alternatively the inductance l2 may be resonant to a wave length above the longest receiving wave length. If the input circuit is untuned; that is, when omitting the condenser [3 the resonance curves obtained will not be exactly symmetrical, the attenuation being greater on one side of the resonance characteristic. This discrepancy can be minimize-d by increasing the degree of inverse feedback or tightening the coupling between the coils l2 and 26.

Referring to Figure 5, there is shown an embodiment for obtaining a band-pass filter having an extremely sharp cut-off by the employment of selective inverse feedback according to the invention. In this modification, item 43 represents a source of input signals which may be an antenna circuit, transmission line, etc. In the example illustrated representing a superheterodyne receiver, the input signal energy is applied to a frequency changer or mixer 45 of any known type through an input transformer 44, whereby the signal frequency is changed to an intermediate frequency. The intermediate frequency energy supplied by the mixer 45 is impressed simultaneously upon the input grids of a pair of amplifying valves 48 and 49 through a transformer 46 having its secondary tuned to the intermediate frequency by means of a condenser 4']. The amplified output energies of valves 48 and 49 are transmitted through filters 50 and 5| and impressed upon a pair of further amplifying valves 52 and 53, respectively. The output of valve 53 is fed to a tuned circuit comprising an inductance 56 shunted by a condenser 51 and serving as a coupling element whereby amplified signal energy may be derived from the output terminals a and b, the latter being connected to the anode of valve 53 through a coupling or blocking condenser 58. The output of valve 52 is similarly fed to a resonant circuit comprising a condenser 55 shunted by an inductance 54 having an inverse feedback coil 59 connected in series therewith, coil .59 being arranged in inductive coupling relation with the input transformer 45. The inductances 54 and 56 of the output circuits are mutually coupled with eachother and the circuits are preferably of a highly damped type.

It can be shown that in an arrangement of the afore-des'cribed type by proper design of the valv coupling filters 56 and 5| to have phase characteristics P1 and P2 displaced relative to each other as shown in Figure 6 (relative inputoutput phase (p plotted against frequency) frequency response curves may be obtained as shown at G in Figure 7 for the circuit 54, 55, 59 and at H in Figure 8 for the circuit 56, 51. As a result all frequencies will be attenuated in the input circuit by the effect of inverse feed-back except for a selected band as shown in Figure 7 in such a manner as to obtain a resultant input-output characteristic for the entire amplifier system as shown by curve K in Figure 9.

From the foregoing, it will be obvious that preferably supplied from av valve different from the inverse feedback valve. The rejector circuit may be connected either in the manner shown in Figures 1 and 4, or at any otherpoint of the inverse feedback path as is understood. The

latter in the arrangement illustrated is preferably wound with thin wire directly upon the coil l2 to ensure maximum coupling between the coils. The number of turns of coil 26 is preferably somewhat lessthan the number of turns of the coil l2. If more than one tuned circuit is used, the circuits may be connected or ganged for .unicontrol in an easy manner due to the fact that a sharp tuning is required only for the rejector circuit in the inverse feedback path, while the other circuits may tune broadly or have a fiat resonance curve and serve merely to increase the amplification rather than for improving the selectivity. It is further understood that several circuits of the type shown may be connected in cascade to still further increase the selectivity.

In addition to improved selectivity as described, the inventive circuit benefits by the various advantages effects of inverse feedback such as increased operational stability, reduced distortion and low noise level, thus'inturn enabling the attainment of higher amplification with a reduced number of circuit elements or amplifying stages and compensating fully or in part for the loss in gain due to'inverse feedback.

In the previously described embodiments the main amplifying and the inverse feedback circuits are arranged in parallel with a common input such as an antenna circuit. According toan improvement as shown in Figures 10 to 13, the inverse feedback and main amplifying valves are connected in cascade by employing the rejector circuit in the inverse feedback path as a coupling element to supply controlling potential for the succeeding amplifier stage. Arrangements of this type have advantages over the previous embodiments as will become more apparent as the following description proceeds.

Referring to Figure 10 there are shown two rejector circuits or wave traps 60 and BI in series with the antenna condenser II and a pair of input inductances 62 and 63. One of the rejector circuits such as 60 may serve for the reception of short waves and the circuit 6| may serve for receiving longer waves. As shown, the long wave rejector circuit 6| and the long wave coup-ling coil 63' may be short circuitedby means of a wave switch 64. The coils 62 and 63 correspond to thecoil. l2 in Figures 1 and 4 and form the primaries of a pair of input coupling transformers having secondaries 65 and 66, respectively, connected in series and across the grid cathode path of a first amplifying valve HI through a grid coupling condenser H and grid leak resistance 12. The anode of valve I0 which serves as a high frequency amplifier is connected to the pole of a high tension source or B-supply through a high frequency coupling choke l3 and voltage drop resistance 14, the latter being shunted by by-pass condenser 15. The anode of valve 10 has further connected thereto an inverse feedback circuit comprising blocking condenser 16, feedback coils T! and 18 in inductive coupling relation, respectively, with input coils 65 and 6B, and a rejector circuit comprising inductances 8i and 82 in series shunted by a variable condenser 83 in a manner substantially similar as shown in Figures 1 and (l. The circuit 8|, 82, 83 besides its function as a rejector in the negative feedback path servesas a coupling network for impressing signal potential switches.

upon the grid of the succeeding valve 85 through grid coupling condenser 88 and grid leak resisttance 81. The valve 85 in the example shown functioning as a detector has its anode connected to the pole of the B-supply through a high frequency choke 88 and coupling resistance 90 a portion of the latter being by-passed for high frequency by condenser 8 I. There is further provided a positive or regenerative feedback path traced from the anode of valve 85 through blocking condenser 92, a pair of feedback coils 93 and 94 in inductive relation with coils 8| and 82, respectively, of the rejector or coupling circuit, further feedback coils 85 and 98 in inductive coupling relation with the input inductances 68 and 65, respectively, for the valve 10, and condenser 81 to ground. Demodulated signal energy is fed from the junction between choke coil 88 and resistance 90 to the input grid of a low or audio frequency amplifier I through coupling condenser I8I and grid leak resistance I02. Item I03 is a self-biasing network comprising a resistance shunted by a condenser in the cathode lead and item I04 represents a transformer in the anode circuit of valve I00 for transmitting the demodulated signals to a further amplifier or utilization circuit. Long wave coils 68, 18, 94, 95 and 82 may be short circuited by means of switches 51, 80, 98 and 84 together with switch 54 by means of a common control element as is customary in receivers embodying multiple wave The input coils 55 and 66 may be shunted by a fixed or variable condenser 68 in a manner and for the purpose similar as described in connection with the preceding embodiments.

The rejector circuits 80 and BI and the antenna circuit have the purpose of suppressing strong local signals and were found to be advantageous in view of the fact that the selectivity of the circuit shown in Figure increases substantially with a decrease in input signal strength. Under these conditions, the rejector circuits 50 and BI were found to operate very efficiently by completely suppressing strong interfering signals to which they are tuned. It was furthermore found that a larger number of rejector circuits each tuned to a different frequency may be connected in the antenna circuit in the manner illustrated. The antenna coupling condenser I I may serve as a volume control element or additionally as a means for tuning the antenna circuit. If the antenna circuit is tuned in addition to the circuit BI, 82, 83 the signal strength may be increased considerably. This receiver was furthermore found to operate 8 more quietly and with a low noise level by reducing the input signal strength. In the latter case, the negative feedback may cause a considerable reduction or complete elimination of interference. For this reason it is advantageous to employ a tuned antenna circuit and to reduce the input coupling 62-55 or 63-56 to a minimum.

A receiver system embodying a tuned antenna circuit is shown in Figure 11. In the latter, there is arranged in series with the antenna a parallel tuned circuit comprising a pair of inductance coils I85 and I01 in. series and shunted by a variable condenser I05. The coil I01 is coupled with coil I08 in series with the inductance 55 in the grid circuit of valve 10. The inverse feedback potential is impressed through the feedback coil 11 upon the inductance 65 in a manner similar as in Figure 10 while positive feedback is provided through a feedback path from the anode of valve 85 through condenser 92, variable condenser H8 and inductance N0 the latter being in coupling relation with the inductance I06 of the antenna tuning circuit. The remaining part of the circuit is similar to Figure 10 with only one wave range being shown for the sake of simplicity of illustration. The tunable rejector and coupling circuit BI, 83 is preferably connected with the antenna tunin circuit for uni-control such as by a common drive or gang control of the adjustable parts of condensers 83 and I05 as indicated by dashed lines III. A circuit of this type was found to be extremely selective especially when using a loose input coupling between coils I01 and I08. The selectively obtained is equivalent to that of two tuned circuits with extremely loose coupling. It is well known that such circuits have a high selectivity, but that the response or sensitivity is decreased substantially by loosening the coupling so as to render their use impractical. When employing a coupling through a negative feedback arrangement of the type described by the invention it was found that the coupling may be reduced considerably and as a result the selectivity increased substantially without appreciable impairment of the response or decrease of signal strength. This behavior is due to the amplifying and filtering action of the feedback valve and the circuit connected therewith. The input circuit of valve 10 is aperiodic by the provision of fixed condenser 68 shunting the coils 55' and I08 as described hereinbefore. In the latter case the response characteristic of the circuit may be substantially broader than the response of the rejector circuits 8|, 83 thereby greatly simplifying the construction and aligning problems for the circuits. The systems described hereinbefore which constitute extremely efficient and selective band-pass filters may be incorporated as a translation or coupling network in any transmission or receiving system and are not limited to the use in the input circuit of a radio receiver as will be readily understood from the foregoing. Thus, a network or filter system of the type shown in Figures 10 and 11 may serve as an intervalve coupling network in the intermediate frequency section of a superheterodyne receiver in which case the adjustment of the rejector circuit or other tuned circuit to the intermediate frequency is fixed resulting in substantial simplification both as regards construction and operation.

In Figure 11, the antenna is coupled to the tuned circuit I05, I05, I01 capacitatively through condenser II, but as is understood it may be coupled in any other manner such as inductively as shown in Figure 13. The selectivity obtained with-a receiver of the type according to Figure 11 using two amplifying stages approximately equals the selectivity of a standard superheten odyne receiver comprising two radio frequency stages followed by two intermediate frequency stages.

If still greater selectivity is desired, a receiving circuit may be used as shown in Figure 13 differing from Figure 11 by the employment of a further tuned high frequency amplifier stage aran intermediate circuit between the antenna and input circuit 65, 68, I08 of valve 10 Thecircuit 65, 08, I08 in the example shown excitesa further high frequency amplifier II8 having its grid connecteddirectly to the high potential side of the circuit 65, 68, I08. The anode of valve II8 isconnected to the pole of the. B-supply through a high frequency coupling inductance I2I n the one hand and to a tuned output circuit comprised of an inductance I22 shunted by a variable condenser I23 through blockingcondenser I20 on the other hand. The tuned circuit I22, I23 is in turn connectedacross the grid cathode path of the detector valve 85 through grid coupling condenser 86 and grid leak resistance8l in the manner similar as shown in Figure 11. The variable condensers 83, I23, I are preferably ganged for uni-control by a common tuning element indicated at III. Positive feedback or regeneration is. provided from the anode I of valve 85 to the tuned intermediate circuit I05, I06, I01 through coil H0 coupled withthe inductance I06. There is further provided a variable condenser I09 between the lower endof the feedback coil IIO and ground for controlling the amount or degree of feedback.

In receiving arrangements with combined negative and positive feedback as shown in Figures 10, l1, 13, the positive feedback or regeneration from the detector valve to the input or antenna circuit operates very smoothly and with great stability in contrast to the known regenerative amplifiers employing positive feedback or regeneration only. Thus, the employment of combined negative and positive feedback in the manner described ensures both increased signal strength and operational stability while at the same time greatly improving the selectivity and minimizing or suppressing distortion and other undesirable defects inherent in the ordinary radio receiver at present being used.

In the arrangements according to Figures 10', 11 and 13 there is provided a fixed condenser 68 connected across the input inductance of the valve 70. As pointed out hereinabove, positive feedback and oscillationsmay occur in case of very tight coupling of the inverse feedback coils I1 and I8 and in order to avoid this defect it has been found advantageous to provide a shunt condenser 68 of fixed capacity of the order between 200 to 400 mrnf. across the input inductance for receivers designed for the broadcast range. The negative feedback causes the circuit comprised by the coils 65, 66, in Figure 10, or 65, I08 in Figures 11 and 13, and the condenser 68 to beproving the selectivity in the manner described has the further advantage of lessening background noise and other interference usually very annoying during reception of radio broadcasts especially in the case of extremely sensitive receivers or when receiving weak distant stations. The reduction of the background noise is the greater, the looser the coupling of the antenna or the smaller the input energy applied to the receiver. In addition, the-negative-feedback provides a certain automaticselectivity control' -by flattening the response curve for-stronger signals andby tightening it in when receiving-weaker signals.

Referring to Figure 12, there is shown an alternative method ofobtaining-selective negative feedback for tuning a radio receiver or other high frequency circuit. Accordingto this modification the inverse feedback path includes an'induction coil 1! coupled with the input inductance 62 so as to oppose the current flow in the latter similar to the previous embodiments; whilethe inverse feedback eifect for the desired frequency-is cancelled by the provision-of a-seriestuned circuit comprising an inductance '-I I2 and avariable condenser II3 shunted across the anode-cathode path of thevalve I0 in series-with a blockingcondenser H4. The output terminalb in the example illustrated is connected to the common junction'point between the inductance and condenser of the series tuned circuit through cou--' pling condenser I I5.

In the modification according to- Figure 12 the series tuned circuit H2, H3 provides'a short circuit for the desired signal which is thus prevented from passing through the inverse feedback coil 11, thereby cancelling the inverse feedback eifect for the desired frequency to which the circuit II2, I I3 is tuned. The valve'IIJ may serve as an inverse feedback valve only in which case the desired signals may be derived from the inductance 02 and applied to aseparate amplifier. or the output signals may be derived from theterminal In either as high frequency or as demodulated signals according to the example illustrated.

As pointed out hereinbefore, the tunable rejector circuitplaced in the inverse feedback path in Figures 10, 11 311C113 simultaneously serves as a coupling element for the input or grid circuit 65,06, 68, Figure 10, or 65, 68, I08, Figures 11 and 13, and accordingly a further feature of the invention consists in the utilization of this effect to provide a multi-stage receiver embodying a plu rality of circuits all being simultaneously tuned to a desired receiving station by means of a limited number including a single tuning element.

An arrangement of the last mentioned type is shown in Figure 14 wherein the input circuit, first negative feedback valve I0 and the outputor detector valve I00 are similar to those shown in the preceding modifications.- In accordance with the improved feature, valve is utilized as a second negative feedback stage and to this end has a feedback path connected between its anode and cathode including a'blocking condenser I25 and a parallel circuit consisting of a condenser I26 shunted by inductance 'I2'l' the .latter being coupled with an induction coil I29 insertedfin series with the grid circuit of this valve! The tunable rejector circuit for this feedback valve is common with the rejector circuit for the valve I0 comprising the inductance BI and variable condenser 83 and is connected-both in the grid circuit of the valve 05 and in the negative feedback path for the valve 10 in the manner described hereinbefore. In this manner the rejection of the desired frequency in both inverse feedback stages is efiected by a single circuit 8|, 83 effective in tuning both feedback circuits. In order to provide additional positive feedback there is provided in the example shown a coupling coil I30 in the cathode lead of valve 85 coupled with the coil 8I of the rejector circuit. Iteml 3I is a self-biasing network'in the cathode lead to provide suitable grid biasing potential in a manner well known.

Referring to Figure 15 there is shown a modification of a resonant multi-stage inverse feedback amplifier comprising a single tuning circuit and differing from Figure 14 by utilizing the valve Ill both for supplying inverse feedback potential and for exciting the succeeding valve 85 through a circuit comprised of an inductance of the tunable rejector circuit. In this manner therejector circuit is effective in suppressing the desired frequency in both inverse feedback circuits thereby affording a control of both amplifying stages by a single adjusting element. The coupling between coils 8| and MI is preferably made variable for adjusting it to an optimum to prevent oscillations through a regenerative path traced from the anode of valve 85, feedback circuit I39, I46, I4I, rejector circuit 8|, 33, coil 'I'I, coil 65 to the grid of valve I0.

Referring to Figure 16 there is shown a multistage receiver constructed according to the invention and embodying a single resonant circuit for simultaneously tuning a plurality of amplifying stages to a desired transmitting station. In the drawings, the first valve I is provided with an inverse feedback circuit comprising blocking condenser I6, feedback coil TI and rejector circuit 8|, 83 which latter in this case is fixedly tuned to an average frequency aboveor below or within the frequency range of the receiver. The rejector circuit 8|, 83 excites the input grid of the next succeeding amplifier III which i in turn provided with a negative feedback circuit including coupling coil I1 coupled with the coil 8| of the preceding rejector circuit. The remaining elements are similar to the first amplifying stage and primed numerals. In this manner any desired number of additional amplifying stages may be provided until obtaining a final stage Inn comprising a tunable rejector circuit 8|n, 8312 arranged in its negative feedback path and serving as an input coupling element for the detector valve 85. Positive regeneration may be provided in any of the previously described manners and from any desired amplifying stage from a higher oscillation level to a point at lower oscillation level such as from the plate of the detector valve 85 through blocking condenser 92, coupling coil III), and regulating condenser I09 shown in the'example illustrated.

In a system of the afore-described type if the circuit 8In, 8311, is tuned to a desired frequency all the preceding circuits 8| (n-l), 83 (n-1) 8|, 83 will be simultaneously tuned although more broadly to the same frequency, thereby causing selective amplification and improving the selectivity of the entire system in the manner as is obvious from the above. Experiments have shown that most favorable and efiicientoperation of the system is obtained by tuning the rejector circuits 8|, 83 8| (n-l), 83 (n-l) to a frequency in the center of the frequency range for which the receiver has been designed. The range above and below this frequency withinwhich all circuits may be simultaneously tuned by th tuning of the circuit am,

identified by corresponding 8312. is somewhat limited if extremely high selectivity is desired as the selectivity is decreased by tuning beyond these limit points. As is understood, all rejector circuits may be variable and ganged for simultaneous tuning in which case thesystem has the advantage over the known multi-stage amplifiers that the alignment of the several circuits does not have to be highly exacting as required in conventional receivers to ensure maximum selectivity.

In the circuits described herein, the effect of inverse or negative feedback on the selectivity or other characteristics of the receiver was found to increase directly in proportion to the amount or degree of feedback used. The negative feedback was found to be accompanied by a slight positive feedback in most cases, thereby providing a limit for the amount of negative feed-back to ensure efficient and stable operation. If a high degree of negative feedback is desirable re sulting in the system being liable to become regenerative and to start to oscillate, this drawback may be avoided by the employment of known means such as neutralizing circuits, by suppressing the effects of interelectrode or other spurious couplings responsible for the undesirable regeneration or tendency to-oscillate.

It will be apparent from the above that the invention is not limited to the specific circuit arrangements shown and disclosed herein, but that the underlying concept and principle of the invention are susceptible of numerous embodiments and variations coming within the broad scope and spirit of the invention as defined by the appended claims. The specification and drawings are accordingly intended to be regarded in an illustrative rather than a limiting sense.

I claim:

1. A selective signal translation system comprising a main amplifying path and an auxiliary amplifying path, each of said amplifying paths having a relatively broad frequency response, means for impressing input signal energy upon both of said amplifying paths, a feedback path for feeding back output energy from said auxiliary amplifying path in inverse phase upon the input of said main amplifying path, and adjustable selective impedance means having a relatively narrow frequency response in said feedback path to suppress the feedback for signals of predetermined frequency.

2. A tuned radio frequency amplifier comprising a main amplifying path and an auxiliary amplifying path both having substantially constant response over an extended range of operating frequencies, means for impressing input signal energy upon both said amplifying paths, a feedback path for feeding back output energy from said auxiliary amplifying path in inverse phase upon the input of said main amplifying path, and a rejector circuit tuned to a predetermined frequency within said range serially inserted in said feedback path.

3. A tuned radio frequency amplifier comprising a main amplifying path and an auxiliary amplifying path both having substantially equal response over an extended band of operating frequencies, means for impressing input signal energy upon both said amplifying paths, a feedback path for feeding back output energy from said auxiliary amplifying path in inverse phase upon the input of said main amplifying path, a rejector circuit tuned to a predetermined frequency inserted in said feedback path, and an additional feedback path for regeneratively feeding back signal energy from a point of higher to a point of lower oscillation level of said main amplifying path.

4. A tuned radio frequency amplifier comprising a main amplifying path and an auxiliary amplifying path both having substantially equal response over an extended band of operating frequencies, means for impressing input signal energy upon both said amplifying paths, a feedback path for feeding back output energy from said auxiliary amplifying path upon the input of said main amplifying path, a rejector circuit tuned to a predetermined frequency serially inserted in said feedback path, and a further feedback path for regeneratively feeding back energy from a point of higher oscillation level of said main amplifying path upon said rejector circuit.

5. A tuned radio frequency amplifier comprising a first amplifying stage having input and output circuits operatively associated therewith and having a substantially constant response over a predetermined band of operating frequencies, a feedback path also having a substantially constant response over said frequency band for degeneratively feeding back signal energy from the output to the input of said stage, a resonant impedance element serially inserted in said feedback path and tuned to offer high impedance to signals having a frequency equal to the signal frequency to be amplified, at least one further amplifier stage, and coupling 'means from said resonant impedance element to the input of said further amplifier stage.

6. A tuned radio frequency amplifier comprising a first amplifier stage having input and output circuits operatively associated therewith and having a substantially constant response over a predetermined band of operating frequencies, a feedback path also having a substantially constant response over said frequency band for degeneratively feeding back signal energy from the output to the input of said stage, a parallel resonant circuit serially inserted in said feedback path and tuned to present high impedance to signals having a predetermined frequency, at least one further amplifier stage, and means for impressing potential developed by said parallel resonant circuit upon the input of said further amplifier stage.

7. A tuned radio frequency amplifier comprising a first amplifier stage having input and output circuits operatively associated therewith and having a substantially constant response over a predetermined band of operating frequencies, a feedback path also having a substantially constant response over said frequency band for degeneratively feeding back signal energy from the output to the input circuit of said stage, a tunable'rejector circuit serially inserted in said feedback path, at least one further amplifier stage, and means for impressing potential developed by said rejector circuit upon the input of said further amplifier stage.

8. A tuned radio frequency amplifier comprising a first amplifier stage having input and output circuits operatively associated therewith and having a substantially constant response over a predetermined band of operating frequencies, a feedback path also having a substantially constant response over said frequency band for degeneratively feeding back signal energy from the output to the input circuit of said stage, a tunable rejector circuit serially inserted in said feedback path, at least one further amplifier stage, means for impressing potential developed by said rejector circuit upon the input of said further amplifier stage, and an additional feedback path for regeneratively feeding back signal energy from a point of higher to a point of lower oscillation level of said amplifier.

9. A tuned radio frequency amplifier comprising a first amplifier stage having input and output circuits operatively associated therewith and having a substantially constant response over a predetermined band of operating frequencies, a feedback path also having a substantially constant response over said frequency band for degeneratively feeding back signal energy from the output to the input circuit of said stage, a tunable rejector circuit serially inserted in said feedback path, at least one further amplifier stage, means for impressing potential developed by said rejector circuit upon the input of said further amplifier stage, and a further feedback path effectively serially including said rejector circuit for degeneratively feeding back signal energy from the output to the input of said further amplifier stage.

10. A tuned radio frequency amplifier comprising a first amplifier stage having input and output circuits operatively associated therewith and having a substantially constant response over a predetermined band of operating frequencies, a feedback path also having a substantially constant response over said frequency band for degeneratively feeding back signal energy from the output to the input circuit of said stage, a tunable rejector circuit serially inserted in said feedback path, at least one further amplifier stage, means for impressing potential developed by said rejector circuit upon the input of said further amplifier stage, a further feedback path effectively serially including said rejector circuit for degeneratively feeding back signal energy from the output to the input of said further amplifier stage, and a third feedback path for regeneratively feeding back signal energy from a point of higher to a point oflower oscillation level of said amplifier.

11. A tuned radio frequency amplifier comprising a plurality of electron tube amplifier stages in cascade, individual feedback paths for feeding back output energy in inverse phase upon the input of each stage, rejector circuits serially inserted in the feedback path of each stage, and coupling connections from each rejector circuit to the input of the succeeding stage, the rejector circuit of the last stage being relatively sharply tuned, and the rejector circuit of the preceding stages being relatively broadly tuned to the frequency of a radio signal being amplified.

12. A tuned radio frequency amplifier comprising a plurality of electron tube amplifier stages, individual feedback paths for feeding back energy in inverse phase from the output to the input of each stage, rejector circuits inserted serially in said feedback paths, [coupling connections from each rejector circuit to the input of the succeeding amplifier stage, the rejector circuit of the last stage being relatively sharply tuned and adjustable over a predetermined band of operating frequencies and the rejector circuits of the preceding stages being relatively broadly and fixedly tuned to predetermined frequencies within said band.

JOZELE' PLEBANSKI. 

